Immune homeostasis is an inherent property of the immune system. It ensures that any antigen- precipitated change of the immune system is kept to the minimum so that the immune responses eliminate the antigen in question but do not lead to fatal inflammatory diseases. The molecular mechanisms through which immune homeostasis is maintained are not fully understood. Using a high throughput gene microarray technology, we recently identified a novel member of the tumor necrosis factor-1-induced protein 8 (TNFAIP8 or TIPE) family, designated TIPE2, which is preferentially expressed in lymphoid and inflamed tissues. TIPE2-deficent mice develop normally, but suffer from spontaneous inflammatory diseases characterized by multi- organ inflammation, splenomegaly, heightened inflammatory cytokine production and premature death. TIPE2-deficient mice are also hypersensitive to septic shock, and TIPE2-deficient cells are hyper-responsive to Toll-like receptor (TLR) and T cell receptor (TCR) signaling. Importantly, TIPE2 binds to caspase-8, and inhibits activating protein (AP)-1 and nuclear factor (NF)-:B activation while promoting death receptor-induced apoptosis. Thus, TIPE2 is a novel anti-inflammatory protein that negatively regulates both innate and adaptive immunity, and its selective expression in lymphoid and inflamed tissues prevents hyper-responsiveness and maintains immune homeostasis. The goal of this research proposal is to define the cellular and molecular mechanisms through which TIPE2 negatively regulates immunity and inflammation. We hypothesize that TIPE2 inhibits TLR and TCR activation by blocking the functions of the activation receptor-induced signalosome (ARIS) that contains Bcl10 (B cell lymphoma 10), MALT1 (mucosa-associated lymphoid tissue lymphoma translocation gene 1), IKK (Inhibitor of NF-?B kinase) and caspase-8. This theory will be tested in models of inflammation at molecular, cellular and organismal levels. The specific aims are: 1) to define the cellular mechanisms through which TIPE2 negatively regulates immunity and inflammation, and 2) to define the molecular mechanisms through which TIPE2 negatively regulates AP-1 and NF-?B signaling pathways.